CRISTIAN asked a question how important is harmony.       I mentioned Peter Kennedy,because he believed it was o k to add [harmony/music]to existing traditional recordings,so he has a relevance as he was a major twentieth century collector.
Cecil Sharp also added harmony to his arrangements for folk songs in schools.
I would agree with frank hamilton it depends,most of us find it acceptable that the Coppers sang in harmony,but may or may not find Sharps arrangements acceptable,and probably dont find Kennedys additions acceptable.
some people[ Chris Roche] of the Shanty Crew dont find singing shanties in harmony acceptable[because he argues it is not authentic]his argument as I understand it,Is that these songs should be sung as they were,without harmony.
   I met peter Kennedy only once,I found him a charming man and feel that overall his contribution was a positive one,.
I have never claimed to be an expert on Peter Kennedy,in fact most of the information, I have provided is easily accessible on the internet.